This disclosure is directed to a formation testing tool and particularly highlights certain methods of operations thereof. After an oil well has been partly drilled and has passed through formations which are thought to be producing formations, one of the next steps in the completion procedure of the well is to perform pressure test on formations penetrated by the oil well. One of the test techniques is to lower a formation testing tool into the oil well. Tests are then performed by making measurements of formation pressure. An exemplary formation testing tool is described in 4,375,164 assigned to the assignee of the present disclosure. As described in that particular disclosure, the tool is adapted to be lowered into the well borehole, supported on an armored logging cable which includes several conductors for providing power to the tool and surface control of the logging tool. The logging cable extends to the surface where it passes over a sheave and is stored by spooling onto a reel or drum. The conductors in the armored logging cable connect from surface control apparatus and power supplies. They also connect to a surface recording system.
One procedure known heretofore is to lower the formation testing tool a specified depth in the well. At that depth, a backup shoe is extended on one side of the formation tester and formation testing apparatus is extended diametrically opposite the backup shoe. The formation testing equipment includes a snorkel system. This involves a surrounding elastomeric sealing pad which isolates an extendible snorkel which penetrates the formation to a specified depth. The snorkel must be isolated from fluid and pressure in the well borehole to be able to measure only the formation. That is, testing of the formation is conducted while isolating the formation tester from fluids and pressures in the well borehole. When the snorkel is extended into the formation, this enables direct fluid communication from the formation into the tool. This permits taking of a sample, and it isolates the sample from invasion of pressure in the well borehole. This permits a sample to be taken free of contamination of other fluids, and it permits pressure tests to be made by means of a pressure sensor to thereby obtain an accurate readout of formation pressure without distorting the data. At the time that a formation test tool is lowered into a well, the possibility of sticking in open hole is always a risk which may result in destruction of the test tool and even the catastrophic loss of the well. It is desirable, therefore, to limit the amount of time that a formation test tool is downhole. It is therefore helpful to speed up operation of the formation test tool.
One limitation is the time required to extend the setting pistons. Through the use of a suitable piston and cylinder arrangement, the above mentioned backup shoe is extended on one side of the borehole so that the formation testing equipment can extend in the diametrically opposite direction. As increase in speed of extension of the backup shoe is helpful, and to that end, the present disclosure sets forth an apparatus which increases setting of the formation test tool. This apparatus incorporates a backup shoe supported by a piston and cylinder for extension, appropriate check valves, a hydraulic circuit connecting with these devices and controlled by two separate three way normally closed hydraulic solenoid valves. Double acting cylinders are utilized so that the backup shoe is extended and retracted under power. The backup pistons and setting piston on which the elastomeric pad is mounted are hydraulically coupled. Therefore, both the backup pistons and setting piston will be extended at an accelerated rate.
An important procedure in execution of pressure test is extending and retracting the snorkel pressure isolated by a surrounding seal. The seal has the form of an elastomeric surface pressed against the formation of interest to isolate borehole fluids and pressures from the formation so that the snorkel can obtain reliable data. This may work for many formations, but in particular, unconsolidated sand formations give difficulty in measurement because it is difficult to perfect a seal against the sand formation. An unconsolidated formation is defined as a sand which washes or erodes with flow. Erosion of sand at or adjacent to the seal face undercuts the seal, and destroys the effective seal accomplished around the snorkel. Even worse, when fluid flow begins through the snorkel, a portion of the unconsolidated formation may flow with the formation fluid. When this happens, the sand erosion at the elastomeric seal around the snorkel may undermine the sidewall of the formation at the snorkel, overcoming sealing isolation of the formation, and thereby permit well fluid to flow into the formation. Unconsolidated sand formations are more likely to produce sand and thus damage the formation shape around the snorkel and the seal when exposed to pressure differentials. The present apparatus incorporates a hydraulic system which maintains a predetermined hydraulic pressure on the supporting elastomeric seal around the seal during sampling to minimize unconsolidated formation sloughing.
With the foregoing in view, the present apparatus is described as an improved formation testing apparatus capable of execution of certain improved procedures. One of the enhanced methods of operation involves speed up extension of the piston and cylinder supporting the backup shoe, wherein formation data can be obtained. Another important procedural advantage of the present invention is the ability of the hydraulically operated control system to sustain predetermined hydraulic pressure acting against the elastomeric seal around the snorkel. More will be noted concerning these and other features of the disclosed apparatus and method of use hereinafter.